Optical networks include nodes interconnected by optical links formed by fiber optic cables including various pre-amplifiers, post-amplifiers, and optional intermediate line amplifiers. Various power control techniques are utilized to control optical power through the various amplifiers, over the optical links. A technique to deal with the control optical power involves the use of so-called channel holders which can include Amplified Stimulated Emission (ASE) sources, modulated lasers, unmodulated lasers, etc. Channel holders are used in optical links to keep optical spectrum in full-fill loading condition so that any capacity change activity can be digitally handled by switching the channel holders with traffic signals. Failure of the channel holder source is expected to be service impacting for existing in-service traffic signals. A power offset (undershoot or overshoot) is expected for existing in-service traffic signals in case of channel holder failure, depending on signals' spectral location, and the ratio of power is being lost due to Stimulated Raman Scattering (SRS), amplifier tilt, Spectral Hole Burning (SHB), and amplifier ripple effects.
To avoid channel holder failure, the typical approach is to place another redundant channel source such that optical power (e.g., ASE power) is combined from both sources, so if one has a failure, then the total optical power only drops by roughly 3 dB, until a local optical controller provides additional power from another source. Disadvantageously, this approach is expensive in cost, power, and real estate as it requires an additional channel holder generator and a passive combiner at every Optical Add/Drop Multiplexer (OADM) switch point for a channel holder-based optical link. Also, this approach does not cover the partial hardware failure case where a part of the spectrum from one source dies due to internal pump failures. In such a case, the second source certainly helps, but it still cannot fully recover the lost power in the spectrum on the line system.
Also, to avoid fiber breaks or pinches between the channel holder source and the OADM multiplexer switch, the channel holders are typically now made in-skin (integrated within the same circuit pack as the OADM multiplexer switch). This approach ensures the fiber and connectors are not exposed outside of the OADM multiplexer switch circuit pack, and reduces any chances for external fiber breaks/pinches.